This invention relates to a system for etching a metal film formed on a semiconductor wafer in a predetermined pattern.
In general, two kinds of metal are used as an electrode or an interconnection film for a semiconductor device: a metal with a high melting point or low resistivity, such as an aluminium alloy, molybdenum or tungsten, and a noble metal of low resistivity, such as platinum or gold. The steps of forming an electrode or an interconnection film by using these metals will be explained below by referring to FIGS. 1A and 1B.
As shown in FIG. 1A, the metal is evaporated, under electron beams or by sputtering, etc., on a predetermined pattern of an SiO.sub.2 film 12 on a semiconductor wafer to form a metal film 13. After a photoresist is coated on the metal film 13, patterning is performed to form a photoresist film as shown in FIG. 1A. Then, the semiconductor structure is immersed in an etching solution to etch the metal film with the photoresist film 14 as a mask. In this way patterning is performed as shown in FIG. 1B. The etching of the metal film 13 progresses with time. In order to properly etch the metal film in a predetermined pattern, it is necessary that the semiconductor structure be raised from the etching solution immediately after the patterning is completed. That is, it is important to know the exact time at which the metal film 13 is etched in the predetermined pattern. The completion of the etching operation is judged either visually or by initially performing tests using test pieces to find a relation of the etching time to the depth of etching and automatically ending an etching operation in an actual etching process on the basis of such a relation. In order to visually determine the depth of etching, it is necessary to raise the semiconductor structure frequently out of the solution to see whether or not the etching process is completed. This process is not only time-consuming, but also complicated. The depth of etching depends greatly upon the temperature of the etching solution. In the automatic etching completion method, it is necessary to control the temperature of the etching solution very accurately, i.e., with an allowable temperature variation range of .+-.0.5.degree. C. Furthermore, the deterioration of the etching solution and a variation in the thickness of the metal film to be etched also complicate the operation.